1. Field of the Invention
The present invention relates to a receiver which utilizes a modulated signal modulated by an angle modulation system or the like to perform communications.
2. Description of the Prior Art
Conventionally, this type of receiver is described, for example, in JP-A-1-122216.
FIG. 1 is a block diagram illustrating the configuration of such a receiver for use in radio communications. In FIG. 1, reference numeral 1 designates an antenna, 11 a high frequency amplifier circuit for amplifying a received signal, 12 a frequency converter circuit for converting the amplified signal to an intermediate frequency signal, 13 an intermediate frequency amplifier, 14 a demodulator circuit for deriving a demodulated signal from the intermediate frequency signal, and 16 an amplitude detector circuit coupled to receive output signals from a plurality of cascade connected amplifiers arranged in the intermediate frequency amplifier 13 for detecting amplitude values of the respective output signals to output a voltage corresponding to an amplitude value of a synthesized received signal.
Incidentally, the intermediate frequency amplifier circuit 13, the demodulator circuit 14 and the amplitude detector circuit 16 may be readily implemented in an integrated circuit (IC).
Also, reference numeral 17 designates a capacitor, 18 a band pass filter and 19 an amplitude detector circuit which detects a change of an amplitude value of a received signal as a voltage together with the other amplitude detector circuit 16.
Further, reference numeral 4 designates a comparator circuit which compares an output signal from the amplitude detector circuit 19 with a threshold value set by a voltage source 5 and controls a variable transmitter circuit 15 to permit transmission, when a good receiving condition is present, to deliver an output signal of the demodulator circuit 14 to an output terminal 3.
Next, the operation of the above-mentioned configuration will be described.
In this example, if the magnitude of a desired wave is sufficiently larger than simultaneously received noise and internal noise generated by the high frequency amplifier circuit 11 and the frequency converter circuit 12, the desired wave itself does not have its amplitude modulated in a predetermined manner, so that the ratio of the desired wave signal to noise signals becomes large, even with these noises, and the amplitude of the desired wave presents only few changes, whereby a low voltage representative of a changing amount of the amplitude value of a received signal, generated by the amplitude detector circuit 19 becomes lower.
However, if the magnitude of a desired wave is small, noise received simultaneously with the desired wave together with noise generated inside the receiver causes the ratio of the desired wave signal to noise signals to become small and amplitude changes to become large, thereby increasing an output voltage representative of a detected changing amount of the amplitude value of the received signal or desired wave. In this event, when the comparator circuit 4 detects that the output voltage of the amplitude detector circuit 19 exceeds a threshold value indicating a predetermined receiving condition, the comparator circuit 4 controls the variable transmission circuit 15 so as not to deliver a demodulated output obtained from the demodulator circuit 14 to the output terminal 3.
On the other hand, when the antenna 1 receives a large amount of noise, even if the magnitude of a desired wave is large, the ratio of the desired wave to noise signals before demodulation as well as the same ratio of a demodulated signal are small, which indicates a bad receiving condition, thereby resulting in a higher output voltage from the amplitude detector circuit 19 representative of a changing amount of the amplitude value of the received signal.
Also in this case, the comparator circuit 4 controls the demodulator circuit 14 for the transmission of a demodulated output corresponding to the threshold value indicating the predetermined receiving condition.
As described above, the measurement of the desired wave signal-to-noise signal ratio is performed utilizing the fact that the amplitude of a desired wave is not modulated.
Other than such influence caused by noise, there are amplitude changes due to a multiple reflex (multi-path) of a wave propagation path, fading and interference from adjacent channels. This type of amplitude change exists largely at a low band and a high band in frequency spectrum from generation mechanism thereof. Also, since this influence does not appear much in an intermediate band (intermediate frequency band), only this intermediate band is extracted by a band pass filter 18.
Thus, even a conventional receiver such as the above described example is capable of correctly measuring and determining a receiving condition and deriving a favorable demodulated signal, even if many noise components are included in a received signal.
However, the above prior art receiver need to assign a lot of channels in a limited frequency range, so that when an intermediate frequency amplifier circuit is set to a narrow bandwidth, signal components of a predetermined angle modulated signal are mixed with a desired wave when a changing amount of the amplitude value of a received signal is to be detected, thereby preventing a correct measurement of a receiving condition.
FIGS. 2A-2C illustrate the relationships between the time and the frequency of respective signals in the receiver shown in FIG. 1. Specifically, FIG. 2A illustrates the relationship between the frequency of a desired wave, i.e., a received signal and the time, FIG. 2B illustrates the relationship between the gain of the intermediate frequency amplifier circuit 13 and the frequency of an angle modulated signal; and FIG. 2C illustrates the relationship between an output signal of the amplitude detector circuit 16 and the time. As for the desired wave or the received signal, since respective band widths of the antenna 1, the high frequency amplifier circuit 11 and the frequency convertor circuit 12 are wide, its gain does not depend upon the frequency so that amplitude modulation is not added to the desired wave.
On the other hand, amplitude modulation of angle modulated signal components can be added to the output signal of the intermediate frequency amplifier circuit 13 set to a narrow bandwidth, as shown in FIG. 2B, whereby the same components are outputted from the amplitude detector circuit 16 as shown in FIG. 2C. These components are too small to cause problems when the intermediate frequency amplifier circuit 13 is set to a wide bandwidth, whereas, when the circuit 13 is set to a narrow bandwidth, such components possibly incur a problem that a correct receiving condition cannot be detected from the output of the amplitude detector circuit 16.